I. Field of the Invention
The present invention is concerned with a process for the production of an oxide ceramic tube which is closed on one end with a top. The tube is made by the extrusion of a plasticized oxide ceramic material through a die and over a spike and then the closing of one end of the tube with a hemisphere-shaped top which is made from the tube material. The invention concerns a device with a die at the end of the extruder and with a spike which is located in the center of the die.
II. Description of the Prior Art
Employing a method which was known before, the extruded tube, which was not baked and open at the end, was closed at one end with a tube top which was separately made from the tube material and die cast. The tube which was closed on one end in the aforementioned way was then baked. In the case of these tubes which were produced according to this method, it was found that the connection between the end of the tube and the top showed a lower strength than the tube itself after the baking process. A separation of the top could occur if the tube were exposed to a higher than normal strain. Attempts have also been made to increase the strength of the connection between the end of the tube and the top by having them made separately and welding them together using an electric arc or a high-temperature oven. After the baking process, however, even this improved method did not show satisfactory results since, even in this case, the top which was welded to the tube did not result in the same strength at the joint as compared with the basic material.
In another improved method it has been attempted to produce the tube with a top at the end in one piece at the beginning of the extrusion process; however, the extrusion process of the entire tube could only be performed in that case when a hole would remain in the top at the end of the tube, through which hole the venting of the inner area of the tube during the tube extrusion was possible. Before the baking of the tube which was produced in such a fashion, the venting hole which remained in the top had to be closed with the unbaked ceramic material by hand. In carrying out the above process, it could be shown that this closing of the venting hole in the top by hand made it possible that the top, which had already its finished form, was distorted in such a way that it would not show the desired wall thickness or shape. In addition, the closing of the venting hole in the top would not insure that the material which was introduced into the venting opening would form a complete bond with the material of the rest of the top. Therefore, even in case of the tubes which were produced according to the latter method, it was found that areas with lower material strength occurred in the general area of the top after the baking process. In the known device which was used to perform the latter method, difficulties were experienced concerning the production of the top at the end of the tube which was equipped with a venting hole during the first stage of the extruding process. Due to insufficient venting, it often was difficult to withdraw the central spike from the mold. The central spike in the present invention is necessary for the formation of the venting hole after the edge of the top has been formed. In many cases a part of the formed top is damaged when the central spike is removed. This made it necessary to close a larger area of the preshaped top by hand than it would have been actually necessary for the venting of the inside of the tube during the subsequent extrusion process. This was also a reason why tube tops resulted which exhibited, in many cases, insufficient strength.